checkBody

Added a `checkBody` function that will ONLY look at `req.body` instead of all of the available params via `req.param()`.

## Motivation
There could be situations where there is a query string value and a body parameter and the user may only want to validate `req.body` and ignore any of the other inputs. 

The `checkParam` function is essentially a copy of what was in `req.check` with some extra parameters around it.